The present invention relates to a drive mechanism for a power tool, and to a power tool incorporating such a mechanism. The invention relates particularly, but not exclusively, to a drive mechanism for a power tool for two-handed use such as a hedge trimmer, and to a hedge trimmer incorporating such a mechanism.
Hedge trimmers are power tools for general garden use with an axially reciprocating blade comprising a plurality of teeth disposed adjacent a stationary blade, the teeth of the stationary blade providing a reaction force for the teeth of the reciprocating blade.
Most hedge trimmers are powered by an electric motor housed such that the rotary output shaft of the motor lies along an axis parallel to the axis of reciprocation of the blade. The axis of rotation of the output shaft is therefore generally parallel to the axis of reciprocation of the blade in order to construct the hedge trimmer as compactly as possible. Consequently, it is necessary to convert rotation of the shaft into reciprocating movement parallel to the axis of rotation of the output shaft, in order to provide a driving input for the blade assembly.
A prior art drive mechanism for producing reciprocating motion in a direction parallel to an axis of rotation of a motor output shaft is shown in FIG. 1. A first bevel gear (20) comprises a plurality of teeth (21) inclined at 45° to an axis of rotation X-X. First bevel gear (20) is supported on shaft (22) and rotates in the direction shown by arrow (23). A second bevel gear (24) comprises a second plurality of teeth (25) inclined at 45° to axis of rotation Y-Y. The teeth (21) of the first bevel gear intermesh with the teeth (25) of the second bevel gear, and second bevel gear (24) is mounted on a shaft (26) such that second bevel gear (24) is free to rotate about axis Y-Y. Consequently, rotation of first bevel gear (20) about axis X-X causes second bevel gear (24) to rotate about axis Y-Y in the direction shown by arrow (27). In this way rotation of a gear about a first axis causes rotation of a further gear about a second axis perpendicular to said first axis.
This prior art drive mechanism suffers from the drawback that the 45° bevel gears must be very accurately machined and located relative to each other as the apex of the truncated cone formed by each gear must coincide. Producing gears having teeth arranged at 45° to the axis of rotation and correctly locating the gears in position relative to each other makes the drive mechanism and corresponding hedge trimmer expensive to produce.
Preferred embodiments of the present invention seek to overcome the above disadvantage of the prior art.